Control device for in-wheel transmissions in an electric vehicle

ABSTRACT

A control device for in-wheel transmissions each of which comprises a stepped transmission that transmits power from each motor to each vehicle wheel of an electric vehicle is arranged to alleviate gear-change shock without increasing the number of steps of the in-wheel transmissions. Gear-change of the in-wheel transmissions of the vehicle wheels is effected with a time difference between the vehicle wheels. The change of acceleration produced by the gear change thereby occurs in such a manner that the change of acceleration is dispersed in regard to time, so the gear-change shock is alleviated compared with the case where gear-change of the four wheels is performed simultaneously. Also, in order to prevent impairment of linearity of advance due to imbalance of the propulsive forces on the left and right of the vehicle produced by drop in the vehicle drive force on gear-change, the output of the motor for the other vehicle wheel on the same side, left or right, as the vehicle wheel that is subjected to gear-change of the in-wheel transmission is increased. Also, gear-change of the in-wheel transmissions for one on each of the left and right sides, making a total of two, vehicle wheels could be performed simultaneously, the gear-change of the in-wheel transmissions for the two remaining vehicle wheels being performed simultaneously with a time difference thereafter.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a control device for in-wheeltransmissions, each of which comprises a stepped transmission thattransmits power from each electric motor to each drive wheel of anelectric vehicle.

[0003] 2. Description of the Related Art

[0004] Electric vehicles are previously known in which an electric motoris individually provided for each drive wheel, and the power from eachelectric motor is transmitted to each drive wheel through each ofin-wheel transmissions comprising a continuously variable transmission(see Laid-open Japanese Patent Application No.H10-89439).

[0005] Also, since the cost of employing a continuously variabletransmission is high, consideration has been given to constructing thein-wheel transmissions as stepped transmissions.

[0006] Since the speed range of an electric motor is wide, in the casewhere the in-wheel transmission is constructed of a steppedtransmission, if the number of transmission steps is two, this isfunctionally sufficient, but, in this case, considerable shock isexperienced on gear-change. A multi-stepped transmission i.e. a three orfour stepped transmission is therefore recommended, but this gives riseto problems regarding increased weight and size.

[0007] In view of this aspect, an object of the present invention is toprovide a control device for an in-wheel transmissions such as toalleviate shock but without increasing the number of steps of anin-wheel transmission comprising a stepped transmission.

SUMMARY OF THE INVENTION

[0008] In order to attain the above object, according to the presentinvention, a control device for an in-wheel transmissions ,each of whichcomprises a stepped transmission that transmits power from each electricmotor to each drive wheel of an electric vehicle, comprising controlmeans whereby gear-change of the in-wheel transmissions for the drivewheels is effected with a time difference at each drive wheel.

[0009] When gear-change of the in-wheel transmissions is effected, thevehicle acceleration changes with the change of torque produced by thetorque characteristic of the electric motors, so, if gear-change of thein-wheel transmissions of all the drive wheels is effectedsimultaneously, the amount of change of acceleration of the vehicle as awhole is the total of the acceleration changes of all the drive wheels,and therefore a considerable shock is produced. However, according tothe present invention, the change in acceleration of the drive wheelsproduced by the gear-changes of the in-wheel transmissions is dispersedin regard to time, so the shock is alleviated.

[0010] However, in a four-wheel drive electric vehicle wherein all fourfront and rear and left and right vehicle wheels are drive wheels, ifgear-change is effected with a time difference in respect of all of thein-wheel transmissions for the four vehicle wheels, the resultant of thedrive force of the vehicle wheel that is being subjected to gear-changeand the drive force of the other vehicle wheel on the same side, left orright, as the vehicle wheel that is being subjected to gear-change,falls below the total drive force of the two vehicle wheels on theopposite left/right side due to the drop in the drive force of thevehicle wheel that is being subjected to gear-change, causing a yawmoment to act on the vehicle. In this case, if the output of theelectric motor for the other vehicle wheel on the same side, left orright, as the vehicle wheel that is being subjected to gear-change isincreased when gear-change of each of the in-wheel transmissions of thevehicle wheels is being conducted, the drop in the drive force of thevehicle wheel that is being subjected to gear-change is canceled by theincrease in the drive force of the other vehicle wheel on the same side,so action of a yaw moment can be prevented.

[0011] Also, in the case of a four-wheel drive electric vehicle, even ifgear-change of the in-wheel transmissions is conducted simultaneouslyfor two vehicle wheels in each case, with a time difference, shock isalleviated compared with the case where gear-change is conductedsimultaneously for the four wheels. Thus, if gear-change of the in-wheeltransmissions is conducted simultaneously for one vehicle wheel on eachof the left and right sides, making a total of two vehicle wheels, andgear-change of the in-wheel transmissions is conducted simultaneouslyfor the remaining two vehicle wheels with a time difference thereafter,the action of yaw moment can be prevented.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a diagrammatic plan view illustrating an electricvehicle in which a device according to the present invention isemployed;

[0013]FIG. 2 is a skeleton view showing an in-wheel transmission;

[0014]FIG. 3 is a graph showing the output characteristic of an electricmotor;

[0015]FIG. 4 is a time chart illustrating the change of motor speed,motor torque and acceleration when gear-change is effected with anin-wheel transmission using a device according to the present invention;

[0016]FIG. 5 is a view showing the drive force of each vehicle wheelduring gear-change of an in-wheel transmission of the right front wheel;

[0017]FIG. 6 is a view illustrating the drive force of each vehiclewheel when the output of the electric motor for the right rear wheel isincreased on gear-change of the in-wheel transmission of the right frontwheel;

[0018]FIG. 7(A) is a view illustrating the drive force of each vehiclewheel during simultaneous gear-change of the in-wheel transmissions ofthe right front wheel and left rear wheel and FIG. 7(B) is a viewillustrating the drive force of each vehicle wheel during simultaneousgear-change of the in-wheel transmissions of the left front wheel andright rear wheel;

[0019]FIG. 8(A) is a view illustrating the drive force of each vehiclewheel during simultaneous gear-change of the in-wheel transmission ofthe left and right front wheels and FIG. 8(B) is a view illustrating thedrive force of each vehicle wheel during simultaneous gear-change of thein-wheel transmission of the left and right rear wheels; and

[0020]FIG. 9 is a time chart illustrating the change of motor speed,motor torque and acceleration when gear-change of the in-wheeltransmissions is performed simultaneously for all four wheels.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021]FIG. 1 illustrates a four-wheel drive electric vehicle in whichall four vehicle wheels 1 at the front and rear and the left and rightare drive wheels; the arrangement is such that individual electricmotors 2 are provided for each vehicle wheel 1, and each vehicle wheel 1is driven by each electric motor 2 through an in-wheel transmission 3incorporated in each vehicle wheel 1. The electric motor 2 and thein-wheel transmission 3 for each vehicle wheel 1 are controlled by acontroller 4 comprising an on-board computer.

[0022] The in-wheel transmission 3 is constituted by a steppedtransmission that performs two-step i.e. high and low gear-change. Asshown in detail in FIG. 2, a planetary gear mechanism 32 and a reducinggear train 33 are interposed in series between an input shaft 30 linkedwith the electric motor 2 and an output shaft 31 linked with the vehiclewheel 1. The planetary gear mechanism 32 comprises a sun gear 32 a, aring gear 32 b, planet gears 32 c that mesh with the sun gear 32 a andthe ring gear 32 b and a carrier 32 d that carries the planet gears 32c; the sun gear 32 a is linked with the input shaft 30 and the carrier32 d is linked with the output shaft 31 through a reducing gear train33. Also, there are provided a clutch 34 that links the ring gear 32 bwith the input shaft 30, an LR brake 35 that brakes the ring gear 32 b,and a one-way clutch 36 that prevents reverse rotation of the ring gear32 b. Thus, if the clutch 34 is engaged, the carrier 32 d is put intothe condition in which it rotates with the same speed as the input shaft30 i.e. high gear ratio is established; if the LR brake 35 is engaged,the carrier 32 d is put in to the condition in which it rotates withreduced speed relative to input shaft 30 i.e. low gear ratio isestablished. During reverse drive, the electric motor 1 is rotated inreverse with the LR brake 35 engaged. The clutch 34 and the LR brake 35are respectively engaged/disengaged by actuators 34 a, 35 a controlledby the controller 4, so that the clutch 34 and the LR brake 35 areengaged/disengaged under the control of controller 4 in response to therunning condition of the vehicle, to effect automatic gear-change withtwo gear ratios i.e. high and low. Numeral 5 in the drawings denotes avehicle wheel brake.

[0023] The rated torque of the electric motor 2 changes as shown in FIG.3 in response to the rotational speed Nm of the electric motor. Thevehicle acceleration is decreased in the torque phase of the initialperiod of gear-change when gear-change takes place from low gear ratioto high gear ratio of the in-wheel transmission 3, and the vehicle wheelacceleration is increased by increase in the motor torque on decrease ofmotor rotational speed Nm in the inertial phase. When gear-change of thein-wheel transmissions 3 for all the vehicle wheels 1 is effectedsimultaneously, as shown in FIG. 9, the acceleration changes of all thevehicle wheels 1 are superimposed, causing a considerable sinkingfeeling in regard to the acceleration G of the vehicle as a whole.

[0024] In contrast, in the present embodiment, as shown in FIG. 4,taking the four vehicle wheels 1 in order, it is arranged forgear-change from low gear ratio to high gear ratio of the in-wheeltransmission 3 for each of the vehicle wheels 1 to be performed with atime difference by a control means constituted by controller 4 in theorder: first vehicle wheel, second vehicle wheel, third vehicle wheel,fourth vehicle wheel. In this way, the change of acceleration of thevehicle wheels during gear-change occurs in a distributed fashion inregard to time, so the sinking feeling of acceleration G of the vehicleas whole is reduced to ¼ compared with the case where gear-change of thefour wheels is effected simultaneously; the gear-change shock is therebyalleviated. Tm in FIG. 4 and FIG. 9 indicates the total torque of theelectric motors 2 of the four vehicle wheels 1.

[0025] Also, in this embodiment, the ratio of the inertia of the ringgear 32 b including the outer member of the clutch 34 and the inertia ofthe rotor of the electric motor 2 is set to be equal to the reciprocalof the ratio of the speed change of the ring gear 32 b and the speedchange of the rotor during gear-change. In this way, the energygenerated on deceleration of the rotor during gear-change is completelyconsumed as energy for rotating the ring gear 32 b, so inertial shockduring gear-change is eliminated.

[0026] By the way, the lowering of the drive forces of correspondingvehicle wheels 1 during gear-change of the in-wheel transmissions 3causes a yaw moment to act. For example, as shown in FIG. 5, duringgear-change of the in-wheel transmission 3 for the right front wheel,the propulsive force on the right-hand side of the vehicle, constitutedby the resultant of the drive force Ffr of the right front wheel and thedrive force Frr of the right rear wheel becomes lower than thepropulsive force on the left-hand side of the vehicle constituted by theresultant of the drive Force Ffl of the left front wheel and the driveforce Frl of the left rear wheel, due to a lowering of the drive forceFfr of the right front wheel during gear-change; a yaw moment M in theright-hand direction therefore acts, impairing the linearity of advanceof the vehicle. Consequently, during gear-change of the in-wheeltransmissions 3 of the vehicle wheels 1, it is desirable to exercisecontrol whereby the output torque of electric motor 2 of the othervehicle wheel 1 which is on the same side, in terms of left and right,as the vehicle wheel 1 at which a gear-change is taking place isincreased by using controller 4 to increase the current supplied to thedrive motor 2 in question. In this way, for example when a gear-changeof the in-wheel transmission 3 for the right front wheel takes place,the torque of the electric motor for the right rear wheel is increased,thereby increasing the drive force Frr of the right rear wheel as shownin FIG. 6; by means of this increase, the drop of drive force Ffr of theright front wheel is canceled, thereby balancing the propulsive force onthe right-hand side of the vehicle and the propulsive force on theleft-hand side of the vehicle, so that a yaw moment no longer acts.Also, the inconvenience of a temporary reduction in the propulsive forceof the vehicle as whole during gear-change does not occur. Although thismeans that, in the condition where the electric motor 2 is outputtingthe rated torque corresponding to the motor rotational speed at thattime point, the current of above the rated current will then be suppliedto the electric motor 2 causing it to output the torque of above therated torque, the time for which such control is performed is short, sothere is no adverse effect on the life of the electric motor 2.

[0027] Although hereinabove an embodiment was described in whichgear-change of the in-wheel transmissions 3 was effected with a timedifference in respect of all of the four wheels, it would also bepossible to simultaneously conduct gear-change of the in-wheeltransmissions 3 for one vehicle wheel on each of the left and rightsides, making a total of two vehicle wheels, for example the right frontwheel and left rear wheel and then, with a time difference thereafter,to simultaneous by conduct gear-change of the in-wheel transmissions 3for the left front wheel and right rear wheel. FIGS. 7(A) and (B) showthe drive force of the vehicle wheels when this is done: both in thecase of gear-change of the in-wheel transmissions 3 for the right frontwheel and left rear wheel and in the case of subsequent gear-change ofthe in-wheel transmissions 3 for the left front wheel and right rearwheel, the propulsive force on the right hand side of the vehicle andthe propulsive force on the left-hand side of the vehicle are maintaineduniform, so no yaw moment acts.

[0028] It is also possible to simultaneously conduct gear-change of thein-wheel transmissions 3 for the right front wheel and left front wheeland subsequently with a time difference thereafter to simultaneouslyconduct gear-change of the in-wheel transmissions 3 for the right rearwheel and left rear wheel. In this case also, as shown in FIGS. 8(A) and(B), the propulsive force on the right-hand side of the vehicle and thepropulsive force on the left-hand side of the vehicle are maintaineduniformly, so no yaw moment acts.

[0029] Although hereinabove the description was given for a four-wheeldrive electric vehicle, even in the case of a two-wheel drive electricvehicle in which only the front wheels or only the rear wheels are drivewheels, gear-change shock can be alleviated by performing gear-change ofthe in-wheel transmissions of the two drive wheels with a timedifference therebetween.

[0030] As described above, with the present invention, gear-change shockcan be alleviated by keeping the change of acceleration of the vehicleas whole produced by gear-change of the in-wheel transmissions small, sothere is no need to increase the number of steps of the in-wheeltransmissions in order to alleviate gear-change shock; this makes itpossible to reduce the weight and size of the transmissions.

What is claimed is:
 1. A control device for in-wheel transmissions in anelectric vehicle, each of the in-wheel transmissions comprising astepped transmission that transmits power from each electric motor toeach drive wheel of the electric vehicle, wherein said control devicefor in-wheel transmissions comprises control means whereby gear-changeof the in-wheel transmissions for the drive wheels is effected with atime difference at each drive wheel.
 2. The control device for in-wheeltransmissions in an electric vehicle according to claim 1, wherein allfour vehicle wheels, front and rear and left and right, are drivewheels, and said control means is constructed so that, in the event ofgear-change of each of the in-wheel transmissions for each vehiclewheel, said control means increases the output of the electric motor forthe other vehicle wheel on the same left or right side as the vehiclewheel which is being subjected to gear-change.
 3. A control device forin-wheel transmissions in an electric vehicle, each of the in-wheeltransmissions comprising a stepped transmission that transmits powerfrom each electric motor respectively to the four vehicle wheels at thefront and rear and left and right of the electric vehicle, wherein saidcontrol device for in-wheel transmissions comprises control means thatsimultaneously effect gear-change of the in-wheel transmissions for onevehicle wheel on each of the left and right sides, making a total of twovehicle wheels, and that simultaneously effect gear-change of thein-wheel transmissions for the remaining two vehicle wheels with a timedifference thereafter.